Experimental investigation of the 9.2 and 24.3 keV nuclear transitions in $$^{227}$$ 227 Th

The 9.2 and 24.3 keV nuclear transitions in $$^{\mathrm {227}}$$ Th were studied in the $$\upbeta ^{-}$$ decay of $$^{\mathrm {227}}$$ Ac employing the internal conversion electron spectroscopy. Values of $$(9244.6 \pm 0.8)$$ and $$(24343.1 \pm 1.1)$$ eV were determined for their energies. The 24.3 keV transition was found to be of the mixed (M1 $$+$$ E2) multipolarity with the spectroscopic admixture parameter $$\delta ^{\mathrm {2}}$$ $$(E2/M1)$$ = $$(0.0116 \pm 0.0004)$$ . Energies of $$(24342.9 \pm 1.2)$$ , $$(28613.3 \pm 1.7)$$ , and $$(37860.2 \pm 2.0)$$ eV were obtained respectively for the 24.3, 28.6, and 37.8 keV transitions in $$^{\mathrm {227}}$$ Th by means of the gamma-ray spectroscopy. Natural atomic-level widths of $$(14.1 \pm 0.5)$$ , $$(11.4 \pm 0.5)$$ , $$(6.9 \pm 0.4)$$ , $$(11.4 \pm 1.4)$$ , $$(8.6\pm 1.2)$$ , and $$(6.0 \pm 0.7)$$ eV for the M $$_{\mathrm {1}}$$ -, M $$_{\mathrm {2}}$$ -, M $$_{\mathrm {3}}$$ -, N $$_{\mathrm {1}}$$ -, N $$_{\mathrm {2}}$$ -, and N $$_{\mathrm {3}}$$ -subshells of thorium, respectively, were derived from conversion electron lines. The cross checking of the energy values of the 9.2, 15.1, and 24.3 keV nuclear transitions obtained by the ICES method is also given.